1. Field of the Invention
This invention relates to a liquid composition used together with a pigment ink, which makes use of a pigment as a coloring material, and also relates to an image forming method applicable to an ink jet recording method, a cartridge, a recording unit and an ink jet recording apparatus which make use of the liquid composition.
2. Description of the Related Art
In inks for ink jet recording, images formed using pigment inks, which contain pigments as coloring materials, are superior to images formed using dye inks, which contain dyes as coloring materials, in respect of fastness such as light-fastness and ozone fastness. On the other hand, the images formed using pigment inks have a problem that they are inferior to the images formed using dye inks, in that coloring materials may be scraped off when images are rubbed with fingers or the like, i.e., they are inferior in scratch resistance.
To cope with such a problem, many proposals are made on how to protect recorded images by applying a surface coat onto the images. For example, a proposal is made on that a protective-layer transfer sheet is thermally transferred to images to protect the images (see Japanese Patent Application Laid-open No. 2000-153677; hereinafter “Patent Document 1”). A proposal is also made on that heat and pressure are simultaneously applied to images to protect the images (see Japanese Patent Application Laid-open No. 2003-170650; hereinafter “Patent Document 2”). A proposal is further made on that images formed using pigment inks are provided with a compound having film forming properties, to protect the images (see Japanese Patent Applications Laid-open No. 2004-99766 and No. 2005-81754; hereinafter “Patent Documents 3 and 4”, respectively).
However, scratch resistance required for images displayed outdoors, such as posters, panels, signs and pop advertisements is so high as to be insufficient at all if it is at such a level that, as in conventionally available techniques, any coloring materials do not come scraped off when images are rubbed with fingers or the like.
For example, when posters and advertisements are printed, fairly large recording mediums of, e.g., A0 size and A1 size are often used, and it is common for them to be rolled up into cylindrical shape when recording mediums on which images have been formed are carried. Hence, problems as stated below may arise. When a recording medium is rolled up, images may be rubbed at edged portions such as corners, of the recording medium. On such an occasion, even where conventional pigment inks are used which are deemed to satisfy the scratch resistance of images at a high level, the images formed may be scratched to make coloring materials scraped off. Such a problem may fairly frequently arise. Problems like this may also arise in other circumstances. For example, there is a case in which, when images formed using pigment inks are posted outdoors as posters, the images are strongly scratched with something sharp such as finger nails. In such a case as well, as in the foregoing, such a problem that coloring materials are scraped off may fairly frequently arise.
To cope with such problems, the method disclosed in Patent Document 1 may be used, where what is aimed in the present invention may be achieved if measures are taken such that the protective layer is made to have a higher strength or have a larger layer thickness. However, in the cases of the methods disclosed in Patent Documents 1 and 2, a device or heat roller which performs thermal transfer is required to make apparatus complicated, and hence this is not so desirable. Accordingly, in recent years, it is sought to develop aqueous inks which can obtain images having scratch resistance higher enough than ever to make coloring materials not scraped off even when something sharp such as finger nails come into touch with the images.
As proposals regarding how the scratch resistance of images is improved when images are formed using pigment inks for ink jet recording, many proposals are made besides those disclosed in the above four Patent Documents. However, according to studies made by the present inventors, what is the best that can be given even by the use of any of such techniques is to achieve scratch resistance of a level that images are not scratched when touched with fingers.
For example, in Patent Documents 3 and 4, a top coat liquid or a colorless ink is incorporated with a water-soluble polymer. However, studies made by the present inventors have revealed that, however much the polymer effective in improving the scratch resistance of images is contained in the liquid composition, the scratch resistance of images is improved only to a certain level because the binding force between a recording medium, a polymer and a pigment depends only on the properties of the polymer. That is, in the invention disclosed in Patent Documents 3 and 4 as well, in which the scratch resistance of images is improved relying only on the performance of the polymer, the scratch resistance is at a level not beyond what is classified as conventional one, and has not at all reached the level of scratch resistance that is aimed in the present invention. More specifically, these techniques have not reached the high-level scratch resistance as aimed in the present invention, such that “coloring materials are hardly scraped off even when images are scratched with something sharp such as finger nails under application of such a strong pressure as to scratch a recording medium at its non-recorded areas”. Especially where the content of the pigment solid matter in an ink is 1.2% by mass or less based on the total mass of the ink, the solid matter that functions as a filler is in so small content that it may be difficult to achieve the high scratch resistance as aimed in the present invention by the use of the polymer alone.
The foregoing is apparent also from evaluation methods in Examples in Patent Documents 3 and 4. More specifically, in Patent Document 3, plain paper is placed on images, and a specific weight is put thereon to rub the images. In Patent Document 4, images are rubbed with an eraser. That is, these evaluation methods are those for evaluating the extent of deterioration when friction is given to the images, and can be said to be based on an evaluation level that is fairly gentler than the level of the scratch resistance as aimed in the present invention.
Further, in the invention disclosed in Patent Document 3, the top coat liquid for protecting the images is applied in an amount of two or more times the amount of the ink to be applied, and it takes a fairly long time until it comes fixed. This is apparent also from the fact that in Examples in Patent Document 3 the scratch resistance evaluation test is conducted after 24 hours. Where images are formed on a recording medium having an ink-receiving layer, a problem may also arise such that the recording medium can not absorb the liquid when the liquid is applied in an amount beyond the ability to retain the liquid, of the ink-receiving layer. That is, it can be said to be not practical to apply the top coat liquid in an amount far beyond the amount of the ink necessary for forming images, as in the invention disclosed in Patent Document 3.